Time digital converters, or TDC, are used whenever one wishes to measure and encode accurately the temporal position of an event, or of a plurality of events, represented by electric pulses, relatively to a reference signal, defining the origin of the temporal scale.
TDCs are used for example in the field of particle physics, to measure the transit time of the elementary particles produced during an interaction, in the different active zones of a segmented particle detector.
TDCs also have applications in many other fields in which an accurate measurement of arrival times of electric pulses is required. In particular, but not exclusively, applications of TDCs comprise temporal photon correlation microscopy, optical tomography, electronic component testing, time-of-flight spectroscopy and reflectometry in the time domain.
A well-known method for encoding time intervals is to count electronically the number of pulses of a clock signal and to copy the value of the counter during the event of interest into a register.
A limitation of this method is that the measurement accuracy is limited by the rate of the clock signal. For a resolution of 10 picoseconds, for example, a 100 gigahertz clock signal is required, so that this level of precision can only be achieved with difficulty by this method.
Another known method is to convert the interval to be measured into a proportionally longer interval, for example with a double-ramp converter, in which a capacitor is loaded and then unloaded with two constant currents of different value.
The time required for the voltage at the terminals of the capacitor to return to zero is proportional to the sought time interval and can be measured with a counter whose rate is relatively low. An inconvenience of this method is the relatively great dead time associated to each measured event, so that this method is only applicable with difficulty to multiple and close pulses, such as for example signals generated by detectors of elementary particles (multi-hit events).
Another inconvenience of the above method is connected to the difficulty of realizing current sources that are constant and independent from the ramp's voltage. Any difference of behavior between the two sources will induce conversion errors.
It is also known to transform a time interval into an analog voltage or load signal of proportional value, thanks to a time amplitude converter (TAC), and to then convert the analog signal into a digital signal by an analog-to-digital converter (ADC). This method has however the disadvantage of a complex and delicate calibration. Another inconvenience of the above method is connected to the difficulty of obtaining a voltage ramp that is exactly linear. The non-linearity of the capacity and/or the non-constancy of the current source are the origin of conversion errors. This method further requires a ramp calibration procedure due to the inevitable initial imprecision of the ramp capacity and current.
It is an aim of the present invention to present a time converter that does not have the inconveniences of the prior art.
It is another aim of the present invention to propose a time converter that associates a high resolution and depth, allowing very accurate measurements of extended temporal intervals.
It is another aim of the present invention to allow a time converter to be made whose resolution is programmable and adaptable to the needs of the application.
It is another aim of the invention to make a time converter that does not require a calibration procedure for it to maintain its accuracy.